Operating systems need to be modified to run on soft-mode Xen. Linux was ported quite quickly, Microsoft made their source code available to Xen and the Xen researchers started to wade through that, but the last I heard they were still slogging away. As I understand it, operating systems can run without modification on hardware based virtualisation - and supposedly much faster which stands to reason but I don't know this first hand and there seems to be some contrary evidence??
I've used soft-mode VMware and it's slow. I doubt that hypervisors (e.g. VMware) would make use of the virtualisation hardware at all if it were slower than just ignoring it, but Parallels uses it if it's there.
I have heard of using the virtualisation as a debugging mechanism, with different bits of the same program working in very different contexts in order to flag up memory errors. That is supposedly slow because of all the context switching. But that's not the same thing...
The Wiki page on virtualisation gives a table of current solutions: http://en.wikipedia.org/wiki/C...on_of_virtual_machines
It's comprehensive but not always consistent or up to date. E.g. VMware's software virtualisation, which runs at 80% of processor speed on computationally intensive (i.e. few os calls, hence easy to virtualise) is somehow listed as "Native speed" = 100% processor speed. !??!
Best wishes, Elf